1. Field of the Invention
The present invention deals with rotary vane valves and, in particular, with means for preventing fluid leaks from such valves.
2. Description of the Prior Art
Rotary vane valves are used as an airlock devices for transferring particulate solids between two regions or systems having different pressures. It is generally desirable that leakage be held to a minimum since such leakage can result in changes in pressure in either region or can constitute a hazard or create deleterious effects such as corrosion, contamination, erosion, or systemic loss of efficiency.
Typical rotary vane valves consist of a cylindrical housing, with opposed inlet and outlet openings for material, and a rotating unit within the housing having a number of pockets for transferring material from inlet to outlet, much as a revolving door permits traffic to pass from a warm area to a cold area with a minimum loss of heat. In certain rotary vane valves, the vane rotor is fitted with annular side plates, called shrouds, which are welded to the ends of the vanes. The closer the clearances between housing and rotating unit can be held, the less leakage or loss of pressure there will be.
There are many reasons why the close clearances required are often impractical, costly, and, in certain cases, even impossible to achieve. Particulate material can be trapped or entrained between rotor and housing surfaces, causing binding or abrasion, or both. Temperature gradients between housing and rotor, complemented by the fact that the masses of the two components may be significantly different, can result in differential amounts of thermal expansion and cause binding or seizure. It is also known that high pressure differentials can cause bending forces on the shaft which supports the rotor. If close clearances are used, the result is interference between the rotor and the cylindrical housing. Additionally, as the physical dimensions of such rotary vane valve units increase, the difficulty and cost of manufacture to close tolerances increases significantly.
In order to avoid leakage without the necessity of using such close tolerances, it has been suggested that the cylindrical housing be fitted on each of its ends with gas tight end bells, and that pressurized air of another gas be introduced into the space inside these end bells. This so-called "purge" gas is maintained at a pressure somewhat higher than that which exists across the rotor, and its purpose and effect is to prevent migration of particulate matter being handled in the rotary valve from migrating across the rim of the rotor shrouds into the end bells. Not only would the particulate matter eventually fill up the end bells, but also it can cause severe friction and wear on the rims of the rotor shrouds and on the housing surfaces which face the shroud rims.
If there were no purge applied to the end bells, air or gas which the valve is intended to seal against would tend to flow across the rim of the shrouds into the end cavities, toward the region of lower pressure, i.e., from the discharge port to the inlet port in the case where the discharge port was exposed to greater pressure. At the inlet, the flow would be back across the shroud rims, again carrying particulate matter. The purge not only tends to keep the interface between rim of shroud and housing surface relatively free of particles, but also imposes a limitation on leakage, if it does not curtail it altogether.
The rate of air or gas flow, Q(cfm) follows the theory of flow through orifices and is proportional to orifice area and to the square root of the differential pressure across the orifice. Considering that the orifice in this case is that formed by the clearance between the shrouds and the cylindrical housing, the relationship ##EQU1## is applicable regardless of which direction the gas flow takes. The use of an air or gas purge flow, it can be seen, not only minimizes friction and wear that can be caused by migrating particles, but also reduces leakage.
As the physical size of the rotary valve increases, the manufacturing and operating problems involving close clearances become more pronounced and more costly. The problems of leakage likewise become more significant and the amount of purging necessary to prevent such leakage will also increase. It will be appreciated that increases in purging requirements will also be necessitated as requirements for differential pressure capability grow greater, or as the temperature to which the valve is exposed becomes higher, or as a result of any combination of the above mentioned factors. It will also be appreciated that the purging of the ends bells is an energy-intensive procedure which may also involve a relatively heavy investment in purging equipment. Furthermore, since it is often necessary that a relatively inert fluid such as nitrogen or steam be used as the purging media instead of air, this procedure may also be relatively cost-intensive. It is, therefore, the object of the present invention to provide a rotary vane valve in which leakage of process fluids and particulate material into the end bell areas is effectively controlled while the high costs heretofore associated with purging gas devices are in a large part avoided.